The present invention is directed to a method of removing the sulfur dioxide from a gas stream containing the same by means of absorption using an aqueous absorption medium containing ammonia bearing compounds.
An ammonia-base pulping system like many of the various other chemical pulping systems, yields a residual waste liquor which not only has a significant heating value, but also contains valuable chemicals in such amounts as to justify chemical recovery. A popular method of chemical recovery employed in many types of pulping processes is concentrating the waste residual liquor, incinerating this concentrated liquor and then reprocessing the chemicals recovered to form solutions useful in the pulping process. In applying this technique to the ammonia base pulping process, one must be prepared to deal with the particular amount of sulfur dioxide present in the flue gases.
Removal of sulfur dioxide by absorption processes has been known and practiced for many years. Aqueous solutions of NaOH, Ca(OH).sub.2, CaCO.sub.3 and other solutes have been used with varying degrees of success, expense and maintenance problems. Due to stringent air pollution control regulations, the requirement for an inexpensive, efficient and reliable system has gained re-emphasis.
A practical way to treat the problem is to absorb the SO.sub.2 by using an absorbent containing ammonia so that the resulting aqueous solution may be reused in the pulping process. However, it has been found that this type of system generates a fume of ammonium sulfite which can be submicron in nature and causes serious pollution and potential visibility problems when released to the atmosphere. Therefore, it is an objective of the present invention to absorb sulfur dioxide from a gas stream employing absorbing media containing ammonia bearing compounds without producing a fume. Another objective of this invention is to provide a method by which chemicals can be removed from the residual waste liquor to form the product acid used in the digestion process.